A flux method is known as a method for producing group 13 nitride crystals such as gallium nitride crystals. In the flux method, a mixed melt (flux) containing an alkali metal or an alkali-earth metal and a group 13 element is formed in a reaction vessel, and a source gas such as nitrogen gas is dissolved in the mixed melt to form a supersaturated state. In the mixed melt, spontaneous nucleation of a group 13 nitride crystal occurs, or the group 13 nitride crystal is grown from a seed crystal as a nucleus.
When such a crystal is grown by the flux method, what remains in the reaction vessel after the growth step is a gallium nitride crystal, an alkali metal or alkali-earth metal, a group 13 element, and an alloy composed of the alkali metal or alkali-earth metal and the group 13 element. To take out the group 13 nitride crystal from the reaction vessel, the remaining alkali metal or alkali-earth metal, group 13 element, and alloy composed of the alkali metal or alkali-earth metal and the group 13 element need to be removed.
When the alkali metal or alkali-earth metal and the group 13 element that have been used as the mixed melt are collected and reused, productivity can be increased in mass production. Thus, what remains needed in the art is to collect the mixed melt in a reusable state. With regard to producing gallium nitride (GaN) crystals by using sodium (Na) as an alkali metal and gallium (Ga) as a group 13 element, the following methods are disclosed that relate to removal of sodium, gallium, and Ga—Na alloy that remain in the reaction vessel at the end of the crystal growth.
Methods for removing sodium from the reaction vessel are known. In such methods, sodium is removed by ionization of sodium by adding alcohol (ethanol in most cases) or water that reacts with sodium (when ethanol is added, sodium ethoxide is formed, and when water is added, sodium hydroxide solution is formed). Such removal methods, however, are highly reactive and there is a risk of igniting the alcohol or causing explosion of hydrogen gas. Patent Literature 1 discloses a method for controlling the temperature of alcohol and water to ensure safety. Patent Literature 2 discloses a method for separating and collecting sodium in a liquefied state. In this method, sodium is heated to a temperature above its melting point and is melt in a medium such as kerosene that is unreactive to sodium.
Methods for removing gallium from the reaction vessel are known. In such methods, gallium is removed by ionization of gallium by adding a strong acid such as hydrochloric acid, nitric acid, or aqua regia that reacts with gallium. Patent Literatures 3 and 4 disclose methods for removing gallium from crystals by heating the residual gallium in the reaction vessel to a temperature above its melting point (29.8° C.) after removal of sodium from the reaction vessel. Methods for removing Ga—Na alloy (intermetallic compound) are known as disclosed in Non Patent Literature 1. In Non Patent Literature 1, the alloy is removed by ionization of an element constituting the alloy by adding aqua regia that reacts with the alloy.
When gallium nitride crystals are produced by the flux method, raw material efficiency (consumed amount of a group 13 element/initial amount of the group 13 element×100) is about 60 to 95%. As described in Patent Literatures 1, 3, and 4, and Non Patent Literature 1, when a few to several tens of grams of gallium is used, the unreacted gallium is not necessarily collected. However, when several hundreds to thousands of grams of gallium is used to produce a large gallium nitride crystal, collecting and reusing the unreacted gallium can reduce the production cost. Unfortunately, Patent Literatures 1 to 4 and Non Patent Literature 1 do not mention a method for separating and collecting gallium from Ga—Na alloy that remains in the reaction vessel at the end of the crystal growth.
With regard to producing gallium nitride crystals by using sodium as an alkali metal and gallium as a group 13 element, the following methods are disclosed that relate to removal of sodium and gallium that remain in the reaction vessel at the end of the crystal growth.
Patent Literature 5 discloses a method for removing an alloy composed of an alkali metal or an alkali-earth metal and a group 13 element by suction after crystal growth and before solidification of the mixed melt. After this process, a group 13 nitride crystal is taken out. This method, however, is unable to separate gallium and sodium in the alloy. Thus, it is difficult to obtain the precise composition ratio of gallium to sodium in the alloy that has been suction-removed to be reused as a mixed melt.
Patent Literature 6 discloses a method for removing the residual mixed melt by heating a crucible after crystal growth in an inert atmosphere to a temperature above the melting point of the mixed melt. When the mixed melt is removed by using a difference in vapor pressure between gallium and sodium, gallium and sodium can be separated. However, heating the mixed melt to 600° C., which is above the melting point of Ga—Na alloy, can cause leakage of sodium vapor, which poses an ignition or explosion threat, and thus is problematic in safety.
In addition to these problems, a large installation is needed to implement the methods disclosed in Patent Literatures 5 and 6. Thus, it is not easy to collect gallium and sodium separately from the reaction vessel after crystal growth.